Copolymerization of a fluoroolefin with other fluoroolefins or non-fluoroolefins provides a copolymer having a wide range of properties from a resin to an elastomer depending on the kinds and/or contents of the fluoroolefin and other monomers. Therefore, the copolymer of the fluoroolefin has various applications, for example, as a part such as an O-ring, a flange seal, a gasket, a diaphragm and a liner and is particularly useful in a field in which excellent resistance to heat and/or corrosion is required.
For producing an elastomeric polymer, a cross linking method plays an important role. Since a copolymer of the fluoroolefin is thermally and chemically stable, it is difficult to cross-link it. For cross-linking the copolymer of fluoroolefin, it has been proposed to copolymerize a monomer which provides a cross-linking site to the polymer. As the monomer providing the cross-linking site, there have been proposed an unsaturated compound having a perfluorophenoxy group (cf. Japanese Patent Publication No. 11823/1972), a nitro group (Japanese Patent Publication No. 26303/1970 and Japanese Patent Kokai Publication (unexamined) No. 61119/1974) or a bromine atom (Japanese Patent Publication Nos. 4115/1978 and 1585/1979). However, it takes longer time to cross-link the copolymer containing such monomer, and the cross-linked product has still unsatisfactory physical properties.
Among the resinous polymers of the fluoroolefins, polytetrafluoroethylene (hereinafter referred to as "PTFE") is most widely used and molded to form various articles. Conventionally, PTFE having a molecular weight of 1,000,000 or more is used for molding. However, since PTFE having such high molecular weight has a large melt viscosity, for example, of 1.times.10.sup.8 PaS at 380.degree. C., its melt processing is very difficult. To decrease the melt viscosity of PTFE, some copolymers of tetrafluoroethylene and other fluorine-containing monomers are proposed and commercially available. These copolymers which contain other monomers in an amount not larger than 2% by mole are called modified PTFE and are processed by substantially the same molding method as PTFE. Examples of the monomer are CF.sub.3 CF.dbd.CF.sub.2, C.sub.3 F.sub.7 OCF.dbd.CF.sub.2, ClCF.dbd.CF.sub.2, C.sub.4 F.sub.9 CH.dbd.CH.sub.2 and mixtures thereof. It is still highly desirable to provide a modified PTFE having further improved performances in molding in comparison with pure PTFE.